which is best for blender rendering cpu or gpu
I’d go pure cpu for now, it’s quite easy to max a gpu’s 4-6gb ram cap

Whilst you can upgrade normal system ram

Plus cpu’s Are closing the gap substantially on GPUs in blender 

To put it into perspective:
Amd’s Threadripper 1950x £800 can render a scene at 90% the speed of a gp100 Nvidia gpu (£4000 card 16gb cap) but if the scene gets over that 16gn limit then and wins hands down 

Check this http://download.blender.org/institute/benchmark/latest_snapshot.html